On Sun, 2013-04-07 at 15:31 +0200, Hannes Reinecke wrote: > On 04/06/2013 11:08 AM, James Bottomley wrote: > > On Fri, 2013-03-15 at 10:46 +0100, Hannes Reinecke wrote: > >> SAM advertises the use of a Well-known LUN (W_LUN) for scanning. > >> As this avoids exposing LUN 0 (which might be a valid LUN) for > >> all initiators it is the preferred method for LUN scanning on > >> some arrays. > >> So we should be using W_LUN for scanning, too. If the W_LUN is > >> not supported we'll fall back to use LUN 0. > >> For broken W_LUN implementations a new blacklist flag > >> 'BLIST_NO_WLUN' is added. > > > > Well, we could do this, but I don't really see the point. By the time > > we get into the report lun code, we've already probed LUN 0, so it's as > > goeod as any for a REPORT LUN scan. > > > Did we? I thought I had avoided that and directly went for probing > W_LUN _first_. > Will be cross-checking. > > > What worries me slightly about the W-LUN is that for the first time > > we'll be assuming a device supports a particular address method > > (Extended Logical Unit addressing) rather than treating LUNs as opaque > > handles we keep and pass back to the target. I appreciate you now have > > a blacklist for failures, but if we didn't use W-LUNs we wouldn't need > > that blacklist. > > > > So could you answer two questions clearly: > > > > 1. What does this buy us over the current LUN0 method? I don't see > > LUN0 might be a valid LUN being a convincing reason. > > LUN masking. > Some HBAs / virtualised devices use LUN masking to forward LUNs to the > virtual machines. > So for LUN0 you have the choice of exposing it to every virtual machine, > meaning you cannot assign a device to LUN0, or have LUN0 as a no-device > LUN which then can be exposed to every virtual machine. That shouldn't matter, should it? The spec says that even a masked LUN must respond to an inquiry (with PQ indicating appropriate inaccessibility). > At which point you run into hardware limitations, as not every storage > array allow for the first option. > And not every LUN masking implementation allows you to expose a single > LUN to several virtual machines. So you might be screwed either way. > > This was the main reason why zfcp could not use the standard LUN > scanning method like every other HBA LLDD and had to resort to manual > LUN activation. So this is an out of spec implementation of LUN masking ... as in it doesn't respond correctly to an INQUIRY? > > 2. What devices have you actually tested this on? > > > Netapp FAS, HP EVA, HP P2000 / MSA, EMC Clariion. > > But as mentioned, I'll be rechecking the patch. > We should _not_ try to probe LUN0 first, but rather send REPORT_LUNS to > the W_LUN directly. If it responds, good. If not, we'll fall back to LUN0. I don't think we can ever do that ... what about SCSI 2 devices that don't support REPORT LUNS or USB devices that will crash on it? We might be able to try a host type whitelist, where if we were a USB or traditional bus host (SPI) we never try this, but if we're a modern one (SAS, FC) we do. James -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
